1. Field of the Invention
The field of the invention is fluidized catalytic cracking of heavy hydrocarbon feeds and cyclones for separating fine solids from vapor streams.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO2 within the regenerator (to conserve heat and reduce air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Most FCC's units are all riser cracking units. This is more selective than dense bed cracking. Refiners maximize riser cracking benefits by going to shorter residence times, and higher temperatures. The higher temperatures cause some thermal cracking, which if allowed to continue would eventually convert all the feed to coke and dry gas. Shorter reactor residence times in theory would reduce thermal cracking, but the higher temperatures associated with modern units created the conditions needed to crack thermally the feed. We believed that refiners, in maximizing catalytic conversion of feed and minimizing thermal cracking of feed, resorted to conditions which achieved the desired results in the reactor, but caused other problems which could lead to unplanned shutdowns.
Emergency shutdowns are much like wheels up landings of airplanes, there is no loss of life but the economic losses are substantial. Modern FCC units must run at high throughput, and run for years between shutdowns, to be profitable. Much of the output of the FCC is needed in downstream processing units, and most of a refiners gasoline pool is usually derived directly from the FCC unit. It is important that the unit operate reliably for years, and be able to accommodate a variety of feeds, including very heavy feeds. The unit must operate without exceeding local limits on pollutants or particulates. The catalyst is somewhat expensive, and most units have several hundred tons of catalyst in inventory. Most FCC units circulate tons of catalyst per minute, the large circulation being necessary because the feed rates are large and for every ton of oil cracked roughly 5-7 tons of catalyst are needed.
Catalyst must be removed from cracked products lest the heavy hydrocarbon products be contaminated with catalyst and fines. Catalyst and fines must also be removed from flue gas discharged from the regenerator. Any catalyst not recovered by the regenerator cyclones stays with the flue gas, unless an electrostatic precipitator, bag house, or some sort of removal stage is added at considerable cost. The amount of fines in most FCC flue gas streams exiting the regenerator is enough to cause severe erosion of turbine blades if a power recovery system is installed to try to recover some of the energy in the regenerator flue gas stream.
The solids remaining at this point are exceedingly difficult to recover, having successfully avoided capture despite passing through several stages of highly efficient cyclones. The solids are very small, essentially all of the solids are below 20 microns, and including significant amounts of submicron to under 5 micron sized material.
Collection of such solids has been a challenge for almost a century. A survey of the state of the art is described in Perry's Chemical Engineering Handbook, in DUST-COLLECTION EQUIPMENT, abstracted hereafter. A gravity settling chamber could be used, but generally only works for particles larger than about 40 microns in diameter. Small particles have a long settling time and are swept out before they settle, unless the device has a large cross- sectional area. The Howard dust chamber improved things a bit by providing multiple horizontal plates in the chamber, so that the dust did not have so far to fall. This device is discussed in Perry's Chemical Engineer's Handbook, Sixth Edition, page 20-82, which reports that the Howard device was the subject of a 1908 patent entitled "Fume Arrester". For an FCC regenerator, with large volumes of regenerator air, and large amounts of fines and dust, a settling chamber with a larger footprint than the entire FCC unit including main fractionator would be required.
Impingement separators improved things a bit, by using inertial forces to drive particles to impinge on collecting bodies in the gas stream. These work well for particles above 20 microns, and have little effect on the dust in FCC regenerator flue gas.
Cyclone separators are settling chambers in which gravitational acceleration is replaced by centrifugal acceleration. FCC regenerators use large cyclone separators, and are able to efficiently recover essentially particles larger than about 15 microns. Collection efficiency is poor for smaller than 15 micron sized particles, and becomes very poor for anything smaller than 5 or 10 microns. To increase collection efficiency in FCC regenerator cyclones, refiners have accepted higher pressure drop by increasing the velocity of incoming gas to the cyclone.
Refiners typically use 2 to 8 primary and 2 to 8 secondary cyclones in their FCC regenerators, because of mechanical constraints and pressure drop concerns. These cyclones have a fairly large diameter, which restricts the amount of centrifugal acceleration which can be achieved.
Thus FCC regenerators inherently let a large amount of fines and dust, in the below 15 micron range, pass out with the flue gas. This material must be removed from the flue gas prior to discharge to the atmosphere, or passage through a power recovery turbine.
Generally a third stage separator is installed upstream of the turbine to reduce the catalyst loading and protect the turbine blades, or permit discharge of flue gas to the air. These can be 20, 50, 100 or even more small diameter cyclones. The third stage separator can use large numbers of small cyclones because it is not in or a part of the FCC regenerator. Small diameter cyclones are used because these give much better fines collection than larger cyclones, for the same gas velocity and pressure drop. Perry's Chemical Engineer's Handbook, Sixth Edition, in Table 20-33 reports that for a 5-20 micron dust mixture, dust collection improves significantly as cyclone diameter decreased, with collection efficiencies for 6, 9 and 24 inch cyclones being 90%, 83% and 70% respectively.
Several vendors (Polutrol and Emtrol) supply systems with many small diameter, horizontally mounted, closely connected and radially distributed cyclones about a central gas outlet. The use of multiple, small, horizontally mounted cyclones is also known for general dust removal, see e.g., the Dustex miniature collector assemble shown in FIG. 20-108 of Perry's Chemical Engineering Handbook, Sixth Edition. Gas is tangentially added to a great number of generally horizontally mounted cyclones. Purified gas is withdrawn via a central gas outlet near the tangential inlet, while dust is removed from the opposite end of the cyclone, which may be of reduced diameter but is unsealed.
Although such third stage separators help a lot, they have never been as efficient as desired, and some refiners have had to install electrostatic precipitators or a baghouse downstream of the third stage separator to reduce fines emissions.
We wanted to improve the operation of third stage cyclones. Based on observations and testing of a small diameter, horizontally mounted cyclone, we realized the way to improve the performance of these cyclones was not to use more or them, or smaller diameter units, but rather to contain some of the problems inherent in the use of such devices.
We observed that the same high velocities, and high centrifugal forces which imparted sufficient inertial force to remove micron and submicron particles also induced turbulence which re-entrained separated particles in the collection chamber. We wanted to retain the virtues of these devices, the high centrifugal forces which could displace submicron particles from a flowing gas stream. To improve the device, we needed to maintain the high energy vortex set up in the collection chamber to induce inertial separation, but prevent this high energy stream from inducing re-entrainment.
We discovered that the operation of small diameter, horizontally mounted cyclones could be improved by providing in the collection chamber of the device a means to permit the high energy vortex extend past the solids outlet. Preferably, a slotted solids outlet is provided which is parallel to the vortex formed in the horizontal cyclone. In this way, we contain the vortex, and protect collected solids from the vortex, thus reducing re-entrainment of collected particles.